1. Field of the Invention
The present invention relates to a molding material containing reinforcing fibers for providing molded articles of high strength, such as a safety shoe toe cap; to a method for producing molded articles employing this material; and to a safety shoe toe cap.
2. Description of the Related Art
Glass fibers, which are commonly employed as reinforcing fibers for synthetic resin molded articles, are available both as short fibers and long fibers. Molded articles incorporating long fibers known in the art include those of random mat configuration in which the fibers are distributed with random orientation within a synthetic resin matrix, those of cloth mat configuration in which the fibers are woven, and those in unidirectional configuration, in which the long fibers are arranged with unidirectional orientation.
Of these molded articles, those containing short fibers or those composed of long fibers in random mat configuration do not satisfy the requirements of JIS standards relating to safety shoe toe caps, for example. Long fiber cloth mat and unidirectional configurations, on the other hand, have problems in terms of molding behavior owing to poor flow properties, and even where successfully molded may in some instances require secondary processing, which contributes to higher production costs.
Referring to FIG. 9, which shows in sectional view a resin safety shoe toe cap material approved for use according to Japanese Industrial standards JIS T 8101, the material comprises in a unitary construction two cloth layers 21 situated at the central portion, and random layers (skin layers) 22 arranged on the surfaces to improve moldability and flow properties.
(i) Shoe toe cap materials include L grade (light work) with compressive strength of .gtoreq.459 kg; S grade (standard work) with compressive strength of .gtoreq.1,020 kg; and H grade (heavy work) with compressive strength of 1,531 kg. For fabrication of resin toe caps, L grade material must incorporate from 2 to 3 cloth layers and S grade material from 4 to 8 cloth layers to reflect the different service requirements; naturally, S grade material, which contains more cloth, will exhibit improved rigidity of the material per se but will also suffer from poor mold insertion behavior, moldabiltiy and flow properties inferior to L grade material, as well as higher defect rates. PA1 (ii) Toe cap materials come in distinct grades, L grade and S grade, with S grade containing a greater number of cloths in the reinforcing layer than L grade, which naturally makes it more expensive. PA1 (iii) Polyamide which exhibits good bonding with the reinforcing fiber is used as the matrix (base material), with reinforcing fiber content typically as high as from 48 to 54%. Moreover, as polyamide per se is water absorbent, and if the roll of material being placed in the mold is not properly set, skin material flow balance vis-a-vis the core material tends to deteriorate during molding, resulting in welds appearing in the molded article and diminished strength on the part of the toe cap. These problems can result in diminished strength due to water absorption and in non-uniform strength in molded articles. PA1 (1) A molding material containing reinforcing fibers, comprising a reinforcing fiber bundle evenly sheathed by a resin layer and having an oblong shape with at least two lengthwise faces that are flat faces. PA1 (2) The molding material containing reinforcing fibers recited in (1), wherein length is from 3 to 50 mm, width is from 1 to 40 mm, and thickness is from 0.1 to 10 mm. PA1 (3) The molding material containing reinforcing fibers recited in (1), wherein reinforcing fiber content is from 45 to 80 wt %. PA1 (4) The molding material containing reinforcing fibers recited in (1), wherein the resin layer consists of a thermoplastic resin selected from polyamide, polybutylene terephthalate, polyphenylene sulfide, polypropylene, and polyethylene terephthalate, and the reinforcing fiber is a fiber selected from glass fibers, carbon fibers, aramid fibers, and metal fibers. PA1 (5) A method for producing a molded article containing reinforcing fibers, comprising the steps of premolding a given quantity of a pellet material of an oblong shape comprising a reinforcing fiber bundle evenly sheathed by a resin layer and having at least two lengthwise faces that are flat faces; and placing the preformed material into a mold and applying heat and pressure to effect molding. PA1 (6) A safety shoe toe cap, produced by assembly and consolidation of an oblong molding material comprising a reinforcing fiber bundle evenly sheathed by a resin layer and having at least two lengthwise faces that are flat faces.
Further, it is necessary to cut the sheet material to the particular dimensions necessitated by the configuration of the molded article, with the cut material then being melted and softened with a far infrared heating oven prior to mold insertion. This process requires mechanical insertion to induce the material to conform to the shape of the mold, so productivity and yield tend to be poor and process control tends to require complicated operations. In some cases, despite good resin impregnation of the reinforcing fibers and satisfactory rigidity, the problem of poorer flex resistance than unimpregnated or semi-impregnated products arises.
To solve problems relating to dimensions when the sheet is subsequently cut and molded, there has been developed a process wherein a composite molding material containing reinforcing fibers oriented in a specific direction is cut to a predetermined configuration, arranged together in a planar configuration, heated, and pressed to effect molding. However, this process involves resin impregnation of the reinforcing fibers from the outset, which leaves unsolved problems relating to flex resistance of the molded article and precludes the possibility of manipulating compression resistance or shock cushioning properties. Further, the composite molding material has considerable length (on the order of 11 mm) in tubular configuration. Materials of this configuration, when weighed and integrated, are difficult to stack in random directions and tend to become oriented in a single direction, with the result that the physical properties of the molded article are strong in the direction of the orientation of fibers but weak in a direction 90.degree. with respect to the direction of the orientation of fibers.